There is an affinity between lubricants and the refrigerants they are used in association with. In addition to the affinity resulting in the presence of lubricant in the refrigerant, additional lubricant is often entrained in passage of the refrigerant through a compressor as a byproduct of compressor lubrication. Commonly, an oil separator is located downstream of a compressor and serves to remove lubricant from the refrigerant with the lubricant being returned to the compressor. Since oil separators are not 100% efficient, some lubricant will get into the system whether or not an oil separator is used. Eventually, oil from the system will be returned to the compressor but there will be some oil "lost" in the system and this is normally addressed in the initial lubricant charge.
Problems can arise where oil discharge from the compressor is not eventually returned to the compressor. One source of such oil loss can be the use of staged compressors with oil separators at the end of each stage. In this case oil passing from the first stage oil separator may be separated out in the second stage oil separator and delivered to the oil reservoirs of the second stage compressors. Actually, any type of oil loss scenario could result in a transfer of oil from the first stage oil system to the second stage.